From nwilson@morgan.com Fri Nov 4 12:20:35 1994 Date: Fri, 4 Nov 1994 10:40:57 -0500 From: Ab Wilson To: analogue@magnus.acs.ohio-state.edu Subject: Re: To all the techie-godz out there. :) mr808@teleport.com writes: > >The Yamaha DX-7 has no filters, but I wouldn't call it "powerful". > > I guess you just don't know how to program it then. Oooh, you bitch... > >Check out the steep resonant bass filter with a slow sweep on the cutoff and > some wacky envelopes controlling the level and resonance. You can't do > that on a Chroma can you. > > Well, not w/ the Chroma, but definitely w/ the Xpander. > > WHY don't more analogues have modulatable resonance??? I'm going to HAVE > to implement this on the Super Jupiter 6... In answer to this question I think it might be because this usually isn't very interresting with a normal filter design althoguh I haven't really explored all the possibilities. Now the real point of this post. Firstly I don't want to slag off either the Chroma or the Xpander. I have a great deal of respect for both these machines. Secondly this is fairly non-analogue but should be of interest to anyone who's into theoretical filter design. Now disclaimers aside, with all due respect you can't emulate the K2k's steep resonant bass filter on either of these machines - not because of anything to do with being able to modulate filter resonance - but because the steep resonant bass filter is a wierd filter design the likes of which I have never seen anywhere else. It acts like a hybrid of a parametric eq, a resonant low pass filter and a shelving bass filter. It has three parameters - resonance (familar to everyone), cutoff fequency (also familar but subtly different in this case), and level - I'll come to that one later. Signals above the cutoff frequency are unaffected, while signals below are either amplified or attenuated to a shelf depending on the setting of the level control. With high resonance and +ve level the response near the cutoff frequency first sharply dips below 0dB to give a notch, comes back up to 0dB at the cutoff, then rises to a peak just above the cutoff before falling back to 0dB for the rest of the response curve. With zero resonance the response is smoother (still 0dB at cutoff) and lacks the notch and peak. The notch and peak get narrower and more pronounced as the resonance increases. If you set a negative level then the response is reversed. Frequencies below the cutoff are attenuated by the level ammount, frequencies just below the cutoff are subject to a resonant peak, and signals above it are hit by a notch before the responce flattens out at 0dB for the rest of the curve. All the slopes in the response curve are very steep - >24dB/oct possibly 36dB/oct. To cut a long story short you can do things with this filter that I can't even do with my modular. Because of the level control you can get the effect of a continuous morph between high pass and low pass which is a very different effect from mixing the outputs of a multimode filter. Because the response above the cutoff frequency is 0dB you can do really screaming high resonance filter sweeps without loosing all the high harmonics (very useful when your source waveform is something other than a standard square/saw etc). You can get some interesting effects by using the resonance control to effect the notch and peak which would be difficult to emulate with other filter designs. In short it makes your standard resonant lowpass filter look quite limited by comparison. I'm not saying digital filters are better than analogue filters here, what I am trying to say (as with my posts on waveshapers) is that analogue designers can learn as much from digital synthesis as digital designers can learn from analogue synthesis. Personally I think resonant low pass filters have been dominant for far too long and it's time that more interesting designs came along (not enough new ideas have gone into analogue synthesis for a very long time IMHO). What I would like to see is someone implement an analogue circuit which has similar characteristics to the filter I've described. (Not me - as I've said before - I only drive them, I don't know what makes them go.) Ab. --